FIG. 10 is a schematic sectional view of conventional acoustic wave element 1 described in patent literature 1. Acoustic wave element 1 is an antenna duplexer complying with the CDMA standard for band 1 for instance, including a receiving filter passing signals in a receiving frequency band from 2,110 MHz to 2,170 MHz and a transmitting filter passing signals in a transmitting frequency band from 1,920 MHz to 1,980 MHz which is lower than the receiving frequency band.
Acoustic wave element 1 includes piezoelectric body 2 made of a piezoelectric material, such as lithium niobate or lithium tantalite, interdigital transducer (IDT) electrode 3 provided on piezoelectric body 2 as an electrode of a resonator of the receiving filter, and IDT electrode 4 provided on piezoelectric body 2 as an electrode of a resonator of the transmitting filter. Wavelength λ501 of an acoustic wave propagating through IDT electrode 3 of the receiving filter is shorter than wavelength λ502 of an acoustic wave propagating through IDT electrode 4 of the transmitting filter.
Thickness T502 of IDT electrode 4 is greater than thickness T501 of IDT electrode 3 to roughly equalize normalized thicknesses N501 and N502, where normalized thickness N501 is obtained by dividing thickness T501 of IDT electrode 3 of the receiving filter by wavelength λ501 of the acoustic wave, and normalized thickness N502 is obtained by dividing thickness T502 of IDT electrode 4 of the transmitting filter by wavelength λ502 of the acoustic wave. This structure improves an electromechanical coupling factor of acoustic wave element 1.
Acoustic wave element 1 includes dielectric layer 5 made of, e.g. silicon oxide (SiO2) provided on piezoelectric body 2 to cover electrodes 3 and 4. Dielectric layer 5 has a temperature characteristic inverse to that of piezoelectric body 2 to improve temperature characteristics of acoustic wave element 1. An upper surface of dielectric layer 5 is flat from above IDT electrode 3 to above IDT electrode 4. In acoustic wave element 1, the transmitting filter including IDT electrode 4 has an inferior temperature characteristic, and the receiving filter including IDT electrode 3 has an inferior electromechanical coupling factor.